Plastic deformation magnesium alloy having excellent thermal conductivity and flame retardancy, and preparation method therefor

ABSTRACT

Disclosed is a magnesium alloy that has high thermal conductivity and flame retardancy and facilitates plastic working, wherein magnesium is added with 0.5 to 5 wt % of zinc and 0.6 to 3.5 wt % of tin (Sn) as a high-melting-point oxide-film-forming element, with, as necessary, 1.5 wt % or less of at least one selected from among calcium (Ca), silicon (Si), manganese (Mn) and mischmetal, the total amount of alloy elements being 2.5 to 6.3 wt %. A method of manufacturing the same is also provided, including melting high-melting-point alloy elements in the form of a master alloy in a magnesium—zinc alloy melt, followed by casting, removing a chill from the cast material, diffusion annealing, and then molding through a tempering process such as rolling, extrusion or forging. This magnesium alloy is improved in ductility by the action of alloy elements for inhibiting the formation of plate-like precipitates in a magnesium matrix structure, can be extruded even at a pressure of 1,000 kgf/cm2 or less due to the increased plasticity thereof, and can exhibit thermal conductivity of 100 W/m·K or more and flame retardancy satisfying the requirements for aircraft materials and is thus suitable for use in fields requiring fire safety, thereby realizing wide application thereof as a heat sink or a structural material for portable appliances, vehicles and aircraft components and contributing to weight reduction.

TECHNICAL FIELD

The present invention relates to a magnesium alloy that has high thermalconductivity and flame retardancy and facilitates plastic working and amethod of manufacturing the same, in which the magnesium alloy isconfigured such that magnesium is added with 0.5 to 5 wt % (hereinafter,% indicates wt %) of zinc such that zinc is solid-solved to thus impartplastic workability, and is also added with 0.6 to 3.5 wt % of tin (Sn)as a high-melting-point oxide-film-forming element, and, as necessary,1.5 wt % or less of at least one selected from among calcium (Ca),silicon (Si), manganese (Mn) and mischmetal, so that the total amount ofalloy elements is 2.5 to 6.3 wt %.

BACKGROUND ART

As known in the art, use of a magnesium alloy, which is the mostlightweight of currently useful metal materials, as a material forvarious metal components in lieu of aluminum is drastically increasingin order to achieve further weight reduction, and demand therefor isconsiderably increasing to address the issue of fuel efficiency ofvehicles and aircraft and application to mobile electronic products.

A magnesium alloy has a density of 1.74 g/cc, which is the lightestamong commercially available structural alloys, and the density thereofcorresponds to ⅔ of the density of aluminum. Furthermore, a magnesiumalloy has superior machinability, high vibration damping capacity, highability to absorb vibrations and impact, and excellent electromagneticwave-shielding performance. The reason why the application of amagnesium alloy to computers, mobile phones, components for vehicles,etc. has recently increased is that it is lightweight and has highrecycling rate and the ability to shield electromagnetic waves, andcasting thereof into a thin profile is possible because castability issuperior to that of aluminum.

However, magnesium has a hexagonal close-packed lattice structure havingnot many slip system, which is essential for plastic deformation, andforming thereof is mainly performed through casting owing to poorextrudability or formability. Here, sand casting makes it difficult toform a desired shape, and die casting causes many problems in thesubsequent surface treatment process because the cast structure thereofis porous. Then, materials such as AZ31, AM20 and the like aremanufactured by alloying aluminum and zinc or manganese, whereby plasticworking using the ductility of a single-phase solid solution becomespossible. However, these materials are developed so as to have a basaltexture after annealing treatment, and a plate or a profile subjected tounidirectional plastic deformation has high anisotropy and makes it easyto form extension twins, and the commercialization thereof is delayedowing to problematic plastic working upon real-world application despitethe high ductility thereof.

Since extrusion at a temperature of 400° C. or more is carried out in atemperature range in which a low-melting-point eutectic liquid phase andan alpha-magnesium solid solution co-exist due to frictional heat with adie, wrinkle-like defects, in which fine surface cracks resemblingfingerprints appear, may occur. Such fine surface defects may decreasefatigue strength and thus must be removed. In actual fields, however,the removal thereof is not easy in terms of cost, environmental factors,and safety issues due to dust ignition. In order to obtain cleanproducts having no surface defects, an extrusion process is performed ata temperature of 350° C. or less. To this end, the processing pressurehas to be increased to at least five times 1000 kgf/cm², which is theextrusion pressure of aluminum.

Moreover, conventional AZ- and AM-based magnesium alloys mainly used fora wrought product are problematic because flame retardancy is notassured due to a low-melting-point eutectic phase.

Such AZ- and AM-based magnesium alloys are disadvantageous in thatcopper (Cu) or high-melting-point iron-based impurities (Fe, Ni) havinglow solubility may form initial precipitates during the solidificationthereof, and a beta-Mg₁₇Al₁₂ compound of aluminum and magnesium, whichis subsequently precipitated, may form coarse plate-like precipitates,and such precipitates are thus interconnected and heat transfer is thusblocked, and thermal conductivity is remarkably lowered even by theaddition of about 3 to 4% thereof (Ed. G. L. Song, Corrosion ofMagnesium Alloys, 2011, pp.137-146).

Thus, when a flame is applied to such a material, it is easy todrastically partially increase the temperature of the heated portion ofthe structure owing to its low thermal conductivity. When it isdissolved, it easily reacts with oxygen in the air and ignites, and evenwhen the flame is extinguished, it is difficult to decrease thetemperature of the material due to slow thermal diffusion and thuscombustion continues, making it difficult to achieve rapidextinguishment, and thus ensuring safety becomes impossible. The concernabout fire affects not only vehicles but encompasses all industries, andthe application of magnesium materials has thus been greatly delayed.

For this reason, many attempts have been made to add rare earth metalsto magnesium in order to impart magnesium with flame retardancy.

Conventional materials therefor may include alloys such as WE43, ZE41,ZE10 or Elektron 21, containing rare earth elements such as yttrium,niobium (Nb), samarium (Sm), ytterbium (Yb), gadolinium (Gd), neodymium(Nd) and zirconium (Zr). These alloys manifest excellent flameretardancy due to a strong oxide film constituted by a rare earthelement but require a large amount of expensive elements or have poorplastic workability, and thus do not adequately satisfy marketrequirements. When the rare earth element is contained in an amount of4% or more, adverse effects in which ductility is remarkably decreasedoccur, and thermal conductivity is generally decreased with an increasein the amount of the alloy element that is added. In particular,zirconium functions to fine the grain size and to increase flameretardancy but has very low thermal conductivity and plasticworkability. Hence, even when zirconium is added in an amount of about1% to magnesium, thermal conductivity may be lowered by 50 to 70%.

As alloys containing zirconium, WE43 has thermal conductivity of 51 to54 W/m·K and ZE41 has thermal conductivity of 24 W/m·K, and these aremainly used as casting materials, rather than plastic working materials,due to the low ductility thereof. Elektron 21 contains about 4% of arare earth element and 0.5% or less of zinc and thus exhibits highthermal conductivity of 116 W/m·K and superior ignition suppressionperformance but very low elongation of about 2%, making it difficult toperform plastic working. ZE10, containing zirconium, has low thermalconductivity and plastic workability and has to be molded through aspecial molding process such as ECAP, making it difficult to actuallyuse in plastic working applications in industrial sites.

Korean Patent No. 10-1367892 discloses a high-temperature magnesiumalloy and a method of manufacturing the same, in which a magnesium alloymelt is added with 0.5 to 3.8 wt % of calcium oxide (CaO), and aluminumand calcium are combined while calcium oxide is reduced, thus impartingflame retardancy. However, this alloy suffers from very low plasticworkability.

Furthermore, a method of increasing thermal conductivity by mixingmagnesium with silicon carbide (SiC) or fibrous alumina has beendisclosed, but plastic workability is deteriorated, and thus the methodis unsuitable for use in a tempering process (A. Rudajevova et al., Onthe Thermal Characteristics of Mg-Based Composites, Kompozyty, 4, 10,2004).

Hence, the production of a magnesium alloy imparted with both flameretardancy and plastic workability is regarded as difficult.

Korean Patent No. 10-0509648, in which plastic workability is increasedby the addition of a rare earth element, discloses a method ofmanufacturing a magnesium alloy plate having superior plasticworkability from a magnesium alloy configured such that magnesium isadded with zinc and yttrium. In this patent, a melt containing 0.5 to5.0% of zinc and 0.2 to 2.0% of yttrium is cast into a plate-likematerial having a thickness of 35 mm, annealed and then rolled into aplate having a thickness of 1.0 mm, thus increasing the plasticworkability of a rolled plate, but center segregation upon casting intobillets having a large diameter of 75 mm or more and zinc gravitationalsegregation, which becomes severe when the zinc content is 3% or more,have not yet been overcome. Furthermore, this patent does not considerimprovements in flame retardancy of the material at all.

In order to increase thermal conductivity and high-temperature stabilityof the material, a conventional AZ-based magnesium alloy is added withan alkaline earth metal such as strontium (Sr) or calcium and combinedwith beta-Mg₁₇Al₁₂ to adjust the shape thereof (A. Kielbus et al., TheThermal Diffusivity of Mg—Al—Sr and Mg—Al—Ca—Sr and Casting MagnesiumAlloys, Defect and Diffusion Forum, Vol. 326-328, 2012, pp.249-254). Thestrontium or calcium functions to increase the surface tension of abeta-phase precipitate to thus suppress the formation of the precipitateinto a lamella phase at grain boundaries, and also, the size of theprecipitate is decreased to thus improve thermal conductivity, and whenthe melt is exposed to flame, a dense surface oxide film is formed andignition is thus prevented. However, this material is composed of 6 to9% of aluminum with 0.8 to 2% of strontium or 1.5 to 2.2% of calcium,the total amount of alloy elements being 8 to 11%, whereby the resultingalloy has low ductility and is unsuitable for use in a temperingprocess. Furthermore, the thermal conductivity of this alloy isincreased by about 75% compared to the thermal conductivity of AZ91, butis still only 87 W/m·K, similar to that of AZ31, and thus does not reachthermal conductivity of 100 W/m·K or more, which is desired in thepresent invention.

Korean Patent No. 10-1276665 discloses a magnesium alloy for plasticworking, in which magnesium is added with 4 to 10% of tin (Sn) and 0.05to 1.0% of calcium (Ca), thus ensuring desired flame retardancy.However, in this patent, since a melt temperature has to be maintainedat 850 to 900° C. in order to dissolve high-melting-point elements suchas calcium, manganese, yttrium, erbium, etc., gas solid solubility andoxide content in the melt are unnecessarily increased, and thus theconcentration of impurities is increased, and moreover, the likelihoodof ignition of the melt is high, undesirably deteriorating workingsafety.

As an alternative thereto, Korean Patent No. 10-1406111 discloses amagnesium alloy composed mainly of magnesium and containing 6.5 to 7.5%of tin and 1% of each of zinc and aluminum.

These alloys are improved in flame retardancy but still exhibit lowplastic workability and thermal conductivity due to the presence of alarge amount of tin, having high precipitation hardenability, and thus,in order to extrude billets therefrom, thermal treatment for a longperiod of time at a high temperature of 480 to 500° C. and an extrusionpressure of 9946 kgf/cm² are required, making it difficult to performplastic working at a pressure of 1000 kgf/cm² or less, which is atypical aluminum extruder pressure in the related industry.

Also, Korean Patent No. 10-0519721 discloses a high-strength magnesiumalloy composed basically of magnesium and 6% of zinc and furthercomprising 0.4 to 3% of manganese, aluminum, silicon or calcium.However, when this alloy is manufactured into commercially availablebillets, the large amount of zinc may cause gravitational segregationand thus billets may break down during extrusion, or plastic workabilitymay decrease, and only high strength and plastic workability arementioned in the detailed description thereof, and no grounds forexpecting good performance in flame retardancy or thermal conductivityare found therein.

In this way, the magnesium alloy has been developed in terms only ofplastic workability or flame retardancy at an early stage, but foractual commercialization thereof, both thermal conductivity and flameretardancy should be satisfied and plastic workability also has to beensured. In order to commercialize the structural material, simplysatisfying only strength and moldability is insufficient, and ignitionof the magnesium material in the event of a fire should be inhibited inorder to prevent the spread of fire and ensure safety.

With the goal of promoting this delayed technological development, theFederal Aviation Administration (FAA), U.S.A., realistically revised thestandards for flammability testing of magnesium alloys for aircraft seatstructures.

According to this regulation (DOT/FAA/TC-13/52) amended in 2014, unlessthe initial weight is reduced by 10% or more in a flammability test fora total of 7 min in which ignition should not occur within 2 min whenexposed to an oil burner flame for 4 min (240 sec) and in whichself-extinguishment should occur within 3 min after the burner is turnedoff, the magnesium alloy meets the performance standard.

FIG. 1 schematically shows a flammability tester for a magnesium alloyfor use in an aircraft approved by the FAA and a test specimen.

DISCLOSURE Technical Problem

Accordingly, the present invention has been made keeping in mind theproblems encountered in the related art, and the present invention isintended to provide a magnesium alloy for use in a tempering process anda method of manufacturing the same, in which the magnesium alloy isconfigured such that magnesium is added with 0.5 to 5 wt % of zinc (Zn)and 0.6 to 3.5 wt % of tin (Sn) as a high-melting-pointoxide-film-forming element, with, as necessary, 1.5 wt % or less of atleast one selected from among calcium (Ca), silicon (Si), manganese (Mn)and mischmetal, so that the total amount of alloy elements is controlledto 2.5 to 6.3 wt %, thus exhibiting superior thermal conductivity andflame retardancy and improving plastic workability, whereby themagnesium alloy may be extruded even at a pressure of 1000 kgf/cm² orless and may manifest superior thermal conductivity of 100 W/m·K or moreand flame retardancy.

Technical Solution

Therefore, the present invention provides a magnesium alloy havingsuperior thermal conductivity and flame retardancy and improved plasticworkability, which is configured such that magnesium is added with 0.5to 5 wt % of zinc and 0.6 to 3.5 wt % of tin (Sn) as ahigh-melting-point oxide-film-forming element, with, as necessary, 1.5wt % or less of at least one selected from among calcium (Ca), silicon(Si), manganese (Mn) and mischmetal, so that the total amount of alloyelements is controlled to 2.5 to 6.3 wt %. Also in the magnesium alloyof the present invention, high-melting-point elements other than zincand tin are added in the form of a master alloy, and mechanicalstirring, suitable for use in preventing chemical segregation, andcasting are performed. Thereafter, a surface chill is removed from thecast material, followed by diffusion annealing and then a temperingprocess such as rolling, extrusion or forging, thereby molding apredetermined profile.

Advantageous Effects

According to the present invention, a magnesium alloy has high ductilityand can be subjected to plastic working without surface defects even ata low extrusion pressure of 1000 kgf/cm² or less, and exhibits superiorflame retardancy and thermal conductivity and can thus satisfy thermalconductivity and flame retardancy required for portable appliances,vehicles, and aircraft components. Such a magnesium alloy extrudate canbe inexpensively manufactured.

DESCRIPTION OF DRAWINGS

FIG. 1 schematically shows a flammability tester and a test specimen;

FIG. 2 schematically shows a stirring process upon cooling a mold;

FIG. 3 shows the high-temperature stable phase precipitation diagram ofAlloy 1 of Comparative Example;

FIG. 4 shows the high-temperature stable phase precipitation diagram ofAlloy 2 of Comparative Example;

FIG. 5 shows the high-temperature stable phase precipitation diagram ofAlloy 3 of Comparative Example;

FIG. 6 shows the high-temperature stable phase precipitation diagram ofAlloy 4 of Comparative Example;

FIG. 7 shows the high-temperature stable phase precipitation diagram ofAlloy 5 of Comparative Example;

FIG. 8 shows the high-temperature stable phase precipitation diagram ofAlloy 6;

FIG. 9 shows the high-temperature stable phase precipitation diagram ofAlloy 7;

FIG. 10 shows the high-temperature stable phase precipitation diagram ofAlloy 8;

FIG. 11 shows the high-temperature stable phase precipitation diagram ofAlloy 9;

FIG. 12 shows the high-temperature stable phase precipitation diagram ofAlloy 10;

FIG. 13 shows the high-temperature stable phase precipitation diagram ofAlloy 11;

FIG. 14 shows the high-temperature stable phase precipitation diagram ofAlloy 12;

FIG. 15 shows the high-temperature stable phase precipitation diagram ofAlloy 13 of Comparative Example;

FIG. 16 shows the high-temperature stable phase precipitation diagram ofAlloy 14 of Comparative Example;

FIG. 17 shows the high-temperature stable phase precipitation diagram ofAlloy 15 of Comparative Example;

FIG. 18 shows the cast structure of Alloy 7;

FIG. 19 shows the cast structure of Alloy 10;

FIG. 20 shows an electron microscope image of the cast structure ofAlloy 7;

FIG. 21 shows the structure of an extrudate of Alloy 6;

FIG. 22 shows the structure of an extrudate of Alloy 7;

FIG. 23 shows the structure of an extrudate of Alloy 10;

FIG. 24 shows the thermal conductivity of Alloy 7;

FIG. 25 shows the thermal conductivity of Alloy 10; and

FIG. 26 shows a flammability test environment.

MODE FOR INVENTION

The present invention addresses a magnesium alloy configured such thatmagnesium is added with 0.5 to 5 wt % of zinc and 0.6 to 3.5 wt % of tin(Sn) as a high-melting-point oxide-film-forming element, with, asnecessary, 1.5 wt % or less of at least one selected from among calcium(Ca), silicon (Si), manganese (Mn) and mischmetal, so that the totalamount of alloy elements is controlled to 2.5 to 6.3 wt %, therebyexhibiting thermal conductivity and flame retardancy, in which thehigh-melting-point alloy elements are added in the form of a masteralloy, thus enabling dissolution at a temperature of 720° C. or less andsuppressing the formation of oxide impurities. When mechanical stirringis performed during the solidification of the melt, segregation of thealloy elements is decreased, and the resulting magnesium alloy mayexhibit superior ductility and plastic working thereof becomes easy. Inparticular, the amount of expensive alloy element is lowered, and thusthe melting point and thermal conductivity may be prevented fromdecreasing and both cost savings and flame retardancy may be satisfied.

In the present invention, the reason why the amounts of alloy elementsthat are added are limited as above is described below.

Zinc is solid-solved in Mg to thus change a c/a axis ratio, therebyinhibiting the development of a basal texture, and uniform plasticworking becomes possible in the solid solution due to the work-hardeningeffects of MgZn₂ or MgZn₅ precipitates. However, if the amount thereofis less than 0.5%, the effects thereof become poor, making it difficultto satisfy work hardenability and ductility required for plastic workingmaterials. On the other hand, if the amount thereof exceeds 5%, thebasal texture, which deteriorates moldability after annealing treatment,may be reinforced, thus drastically decreasing plastic workability.Furthermore, the extent of segregation in the magnesium melt isincreased, and precipitates such as Mg₂Zn₃, Mg₁₂Zn₁₃ and the like arestacked with the alpha phase to thus form a low-melting-point eutecticphase at about 340° C., undesirably deteriorating flame retardancy.Hence, the amount of zinc is preferably set to the range of 1.0 to 4.0%.

Tin is provided in the form of a high-temperature stable phase such asan Mg₂Sn precipitate having a melting point of 560° C. or more and isthus appropriately distributed, thus increasing high-temperature plasticworkability. When it is exposed to flame in air, a SnO₂ oxide filmhaving a melting point of 1600° C. or more is formed to thus contributeto improving flame retardancy. However, in the present invention, if theamount thereof is less than 0.6%, it is difficult to anticipate theeffects thereof. On the other hand, if the amount thereof exceeds 3.5%,the melting point may decrease and the ignition point may be lowered to500° C. or less, and thus flame retardancy may be deteriorated somewhat.Furthermore, ductility may decrease due to the excessive amount of theMg₂Sn precipitate, and manufacturing costs may increase. Hence, theamount of tin is preferably set to the range of 1.0 to 3.0%.

Mischmetal is a rare earth alloy containing about 65 to 78% of cerium(Ce) and lanthanum (La), with the remainder of neodymium (Nd) andpraseodymium (Pr) and inevitable impurities. Mischmetal may exhibit thesame effect as lanthanum in the magnesium alloy, and is inexpensive andmay manifest effects identical to those of lanthanum (La) or cerium(Ce), requiring a refining separation process, and thus may be used inlieu of lanthanum in order to reduce manufacturing costs. When ceriumand neodymium are exposed to flame in air, oxide films such as CeO₂ andNd₂O₃, having respective melting points of 2400° C. and 2200° C. ormore, are formed, and flame retardancy may thus be exhibited. By virtueof the high-temperature stable phase, high-temperature strength ismaintained high, and thus warping, sagging or melt-bar separation athigh temperatures due to the presence of flame may be greatlysuppressed.

However, cerium in mischmetal has low solid solubility limit tomagnesium and the maximum solid solubility limit thereof is 0.5%. Whenelements of mischmetal having large atomic weights are excessivelyadded, coarse precipitates may tend to form. Hence, in the presentinvention, the amount thereof is limited to 1.5% or less. In the presentinvention, the reason why constitutional elements in rare earth metalsare represented as mischmetal is that the addition of individual metalsmay increase manufacturing costs, but the addition of individual metalsdoes not depart from the scope of the invention.

Lanthanum may be used in lieu of mischmetal in the present invention.Lanthanum, which is a typical rare earth metal, has a high solidsolubility limit of 12.4% to magnesium and is combined with a MgZn₂precipitate to thus form a precipitate having a hexagonal long-periodstacking ordered structure so that a lamellar eutectic phase is formedat grain boundaries. The precipitate is converted into a coherentintermetallic compound through spinodal decomposition duringhomogenization heat-treatment after the casting process, and isdispersed through a tempering process and thus contributes to dispersionstrengthening. Upon exposure to flame in the air, lanthanum is formedinto a La₂O₃ oxide film having a melting point of 2300° C. or more, andthus flame retardancy is exhibited. Furthermore, high-temperaturestrength is maintained high due to the high-temperature stable phase,and thus warping, sagging or melt-bar separation at high temperaturesdue to the presence of flame may be greatly suppressed. However, in thepresent invention, a coarse precipitate may be formed with an increasein the amount thereof that is added. Hence, the amount thereof islimited to 1.5% or less.

Calcium, which is a Group 2 alkaline earth metal like magnesium,functions to form a secondary solid phase such as Mg₂Ca having a meltingpoint of 715° C. between dendritic structures, or is solid-solved withinthe matrix structure together with zinc and is thus recrystallized indisordered directions during the heating process, thereby suppressingthe development of a basal texture and fining crystal grains. Whenexposed to flame in the air, calcium is formed into a CaO oxide filmhaving a melting point of 2600° C. or more, and thus flame retardancy isimproved. Furthermore, high-temperature strength is maintained high dueto the high-temperature stable phase, and thus warping, sagging ormelt-bar separation at high temperatures due to the presence of flamemay be greatly suppressed. However, in the present invention, the amountof Mg₂Ca particles is excessively increased with an increase in theamount of calcium that is added, and thus ductility may decrease. Hence,the amount thereof is limited to 1.5% or less.

Silicon is added to form a high-temperature stable phase such as Mg₂Sito thus fine a grain size and exhibit precipitation strengtheningeffects. Although the precipitate thereof may easily become coarse, thesize or shape of the precipitate may be adjusted by the addition ofcalcium. When exposed to flame in the air, silicon is formed into a SiO₂oxide film having a melting point of 1600° C. or more, and thus flameretardancy appears. Furthermore, high-temperature strength is maintainedhigh due to the high-temperature stable phase, and thus warping, saggingor melt-bar separation at high temperatures due to the presence of flamemay be greatly suppressed. However, in the present invention, a coarseprecipitate may begin to be formed with an increase in the amountthereof that is added. Hence, the amount thereof is limited to 1.5% orless.

Manganese has a maximum solid solubility of 2.2% in magnesium. In themagnesium alloy, a peritectic reaction, in which an alpha phase ofmanganese is precipitated, and a monotectic reaction, in which a deltaphase thereof is precipitated, may occur at 650° C., thus fining thegrain size and improving corrosion resistance. In particular, whenexposed to flame in air, manganese is formed into a MnO oxide filmhaving a melting point of 1900° C. or more and thus flame retardancy isimproved. In the present invention, manganese is effective at fining acoarse plate-like Mg₁₇Al₁₂ precipitate that is formed in the presence ofaluminum impurities and also at fining an MgZn₂ precipitate in thecourse of recrystallization during annealing. However, in the presentinvention, the effects thereof are saturated with an increase in theamount thereof that is added, in which case ductility may also decrease.Hence, the amount thereof is limited to 1.5% or less.

In the present invention, 0.5 to 5 wt % of zinc (Zn) and 0.6 to 3.5 wt %of tin (Sn) as a high-melting-point oxide-film-forming element arecontained, and as necessary, at least one selected from among calcium(Ca), silicon (Si), manganese (Mn) and mischmetal is added in an amountof 1.5 wt % or less, thereby yielding a melt, which is then mechanicallystirred during a solidification process. Thereby, ductility may beensured so as to enable extrusion even at a pressure of 1000 kgf/cm² orless, and also, upon exposure to flame, a high-melting-point oxide filmis formed on the surface of material, thus exhibiting flame retardancyand simultaneously satisfying thermal conductivity, ultimatelysuppressing melt-bar separation due to partial heating and shorteningthe self-extinguishing time of the melted-down material. A detaileddescription thereof is given below.

In the present invention, the reason why the total amount of alloyelements is controlled to 2.5 to 6.3% is as follows. If the total amountthereof is less than 2.5%, improvements in ductility and flameretardancy are insignificant. On the other hand, if the total amountthereof exceeds 6%, thermal conductivity may decrease due to theexcessive amount of compounds and precipitates, and ductility and themelting point may decrease. Hence, the total amount thereof is limitedas above.

When a rare earth metal or an alkaline earth metal is added in a largeamount in the related art, ignition is suppressed and thus flameretardancy is improved, but the melting point of the material is loweredand the rate of thermal diffusion is decreased with an increase in theamount thereof that is added. In the event of a fire, when a portionexposed to flame is partially heated, early melt-down may occur and thetime for which the portion remains melted may increase. Hence, in thepresent invention, the total amount of alloy elements is limited to 6.3%or less.

Also in the present invention, even when the total amount of alloyelements is controlled to 6% or less, thermal conductivity may bedecreased in the excessive presence of aluminum or zirconium as animpurity. Even when aluminum is contained in an amount of about 3%, alarge amount of coarse plate-like Mg₁₇Al₁₂ may be formed, and thusthermal conductivity may decrease, and a rare earth or alkaline earthmetal element may be consumed, undesirably deteriorating flameretardancy. Hence, the amount of aluminum as an impurity is limited to1% or less. Furthermore, zirconium is combined with other rare earthelements in the dendritic structure to thus form a lamellar phase atgrain boundaries, thus remarkably decreasing thermal conductivity andincreasing brittleness, undesirably deteriorating ductility. Hence, theamount of zirconium as an impurity is limited to 0.5% or less.

In the present invention, thermal conductivity of 100 W/m·K or more isexhibited through the above method, and at least one selected from amonghigh-melting-point oxide-film-forming elements such as tin (Sn), calcium(Ca), silicon (Si), manganese (Mn) and mischmetal is contained in anamount of 0.3 to 2.0%, thus satisfying flame retardancy in a burnerflammability test, including an ignition time of 120 sec or more, a testspecimen extinguishing time within 180 sec after extinguishment of theburner, and a weight reduction of 10% or less.

In the present invention, in order to decrease the segregation of alloyelements in the melt and improve plastic workability and physicalproperties, high-melting-point alloy elements (calcium, silicon,manganese, mischmetal, lanthanum), are added as the master alloy, withthe exception of low-melting-point elements such as zinc and tin, uponalloying, and the resulting melt is mechanically stirred.

In magnesium alloys, there are many cases in which the difference inspecific gravity between magnesium and other alloy elements is great.For example, zinc or tin is easy to segregate at the center or thebottom of the mold, and dendritic crystals may be coarsely developed,and thus a macroscopic composition of billets may become non-uniform andextrusion performance may decrease. In the case of severe segregation,hot tearing may occur at the center of a billet, and during theextrusion process, deformation, cracking and fine wrinkles may occur,and moreover, the extrudate may break down or may be decreased infatigue strength during the stretching or correction thereof,undesirably deteriorating durability and reliability.

In the present invention, when the high-melting-point alloy elements areadded in the form of the master alloy, the temperature of the melt iscontrolled to 720° C. or less and dissolution may become possible, thusdecreasing the likelihood of ignition of the melt. Furthermore, solidnuclei already formed in the mushy zone in the melt are dispersedthrough mechanical stirring, thus promoting uniform solidification inthe melt, decreasing segregation and fining crystal grains. Inparticular, the magnesium alloy has no unpaired odd electrons and thusdoes not readily exhibit magnetic stirring effects owing to the lowmagnetizing force thereof. Hence, mechanical stirring is effectivelyused.

The specific method therefor is further described in detail in Examples.

The master alloy is produced in a manner in which lumps or grainscomposed mainly of alloy elements are added to a magnesium melt in ashielding gas atmosphere with an air shut-off so as to form acomposition close to a eutectic composition, and mechanical stirring isperformed, thereby lowering the melting point of the master alloy.

Based on the results of tests conducted by the present inventors, whenmechanical stirring is conducted upon gravitational casting of a 30 kgmagnesium alloy billet containing 4% zinc, the total chemicalsegregation difference of the upper and lower portions of the castmaterial is within 1%, but when mold casting is performed but stirringis not conducted, the segregation difference is 8% or more. In order toeliminate such segregation, mechanical stirring is effective. Suchsegregation may cause billets to break down at segregation boundariesupon subsequent extrusion or may remain even after extrusion, resultingin visual defects and non-uniform physical properties.

In the present invention, mechanical stirring is performed using a motorand an impeller. Here, whether the mechanical stirring employs manpoweror an alternative manner does not impact the scope of the presentinvention. In the present invention, when a solidification process isprogressed inside the cast material (e.g. billet) subjected tomechanical stirring, the melt is stirred, whereby a high-temperaturestable phase and high-temperature precipitates formed in a solid phasein a mushy zone in the melt are dispersed and thus function as solidnuclei, and thus the structure of the cast material becomes uniform,segregation is eliminated, and crystal grains become fine.

As shown in FIG. 2, in the present invention, the alloy melt is shieldedfrom air by means of shielding gas injected via a shielding gas pipe 4and a cover 3 provided to a crucible or a mold 1. A stirrer in which asmall impeller 22 is provided to a lever 21 made of stainless steeloperated by a motor M is placed in the melt via the through-hole in thecover during the solidification of the billet, followed by stirring,whereby the alloy composition becomes uniform. Furthermore, the mushyzone 8 is moved vertically, whereby solid nuclei are dispersed and thuscrystals are fined. The impeller 22 useful in the present invention maybe made of another metal, ceramic material, or composite material, ordeposition, plating, infiltration or spray coating of the impeller withanother material may also fall within the scope of the presentinvention. The diameter of the impeller is ⅕ to ⅔ of the billetdiameter. If the diameter of the impeller exceeds the above upper limit,a large motor is required in order to handle the load. On the otherhand, if the diameter of the impeller is less than ⅕ of the billetdiameter, stirring effects become insignificant and are thus inadequateto prevent segregation.

The left side of FIG. 2 shows gravitational casting in which a crucibleis extracted after melting therein and is then solidified, and the rightside thereof shows continuous casting.

Below is a description of the melting in a crucible furnace according tothe present invention.

In the present invention, a magnesium alloy melt is manufactured asfollows. Specifically, a magnesium ingot is first melted in a cruciblefurnace and the temperature thereof is maintained at 680 to 720° C.Here, the crucible is made of stainless steel, and the meltingatmosphere is formed by blocking contact with air while allowing a gasmixture of carbon dioxide gas and 0.25 to 0.3% of SF₆ to flow.Thereafter, among alloy elements, zinc and/or tin are added, and otherhigh-melting-point alloy elements (mischmetal, lanthanum, calcium,silicon, manganese) are added in the form of a master alloy close to aeutectic composition. The melt of the alloy elements that are added asshown in Table 1 below is mechanically stirred and stabilized, afterwhich the crucible is extracted and then sank in a cooling bath. In thisprocedure, the crucible containing the melt is cooled in a manner inwhich a coolant is sprayed, or in which it is sank in a bath containinga coolant such as water at 30° C. or less, thus promoting thesolidification of the melt.

Upon cooling in the bath, the crucible is cooled at a rate of about 70to 200° C./min depending on the capacity of the bath and the temperatureof the coolant. When cooling is performed while the coolant is sprayed,the cooling rate is further increased. On the other hand, when thecrucible is spray-cooled in the bath, cooling is performed at a rate ofabout 200 to 600° C./min. Upon continuous casting, which requires a highsolidification rate, the spraying pressure is increased so that the moldand the billet are more strongly cooled, whereby cooling may beperformed at a rate of about 400 to 900° C./min. If the cooling rateexceeds 900° C./min, the center thereof may crack due to thermalshrinkage stress based on the difference in cooling rate between theinside and outside of the billet.

During the solidification of the magnesium alloy melt, the impeller madeof stainless steel is placed in a crucible and the melt is mechanicallystirred two or three times up and down so that the solid nuclei formedin the cast material are dispersed, thereby obtaining a cast materialhaving low segregation and a fine structure. Through mechanicalprocessing of the cast material, a surface chill is removed to give abillet having a diameter of 74 to 75 mm, followed by diffusion annealingat 380° C. for 2 hr and cooling to room temperature.

Thereafter, the billet subjected to diffusion annealing is preheated at380° C. for 1.5 hr, and the alloys of Table 1 are extruded using anextrusion die, thus forming a plate having a width of 50 mm and athickness of 8 mm.

The alloys of Examples of Table 1 were mostly extruded at a pressure of750 to 900 kgf/cm². However, Alloys 1 and 15 of Comparative Exampleswere extruded, but microcracking was present on the surface of the platedue to the low-melting-point eutectic phase, and Alloy 14 of ComparativeExample was processed under conditions of an extrusion temperaturedecreased to about 340° C. and an extrusion pressure increased to 1500kgf/cm², whereby surface microcracking was prevented but an excessiveself-extinguishing time after removal of the burner in the flammabilitytest and an excessive weight reduction resulted, which are undesirable.In Table 1, the extrusion plastic working was evaluated as follows: thecase where a clean and smooth surface was obtained and the extrusionrate satisfied 1.5 to 2 m/min is represented as O, and the case wherefine wrinkles were formed somewhat on the surface or the extrusion ratewas about 1 m/min is represented as Δ. The respective causes aredescribed in connection with the unsatisfactory cases.

Alloys 2, 3, 4, 5, 13, and 15 of Comparative Examples were unable toundergo plastic working because the cylinder was stopped or the billetwas broken during the processing with an increase in the extrusionpressure in the extrusion process.

Alloy 15 of Comparative Example was extruded under conditions of anextrusion temperature decreased to about 300° C. and a high pressure of5300 kgf/cm². Here, lateral cracking occurred due to excessivebeta-Mg₁₇Al₁₂ precipitation but was removed through processing, thusobtaining a flame-retardant test specimen. However, this test specimenhad a low ignition point and did not meet standards for all of initialignition time, self-extinguishing time and weight reduction in theflammability test.

The plate-like samples thus obtained were subjected to diffusionannealing at 380° C. for 1 hr and processed to a diameter of 12.7 mm anda thickness of 2 mm. Based on the results of measurement of thermalconductivity using a laser flash process in accordance with ASTM E4161,the alloys of the present invention exhibited thermal conductivity of125 W/m·K or more at a high temperature of 100° C. or more.

The chips obtained from the above samples were measured for ignitionpoints through thermogravitational analysis (TGA) using a differentialscanning calorimeter (DSC), and were processed into test specimenshaving a width of 38.1 mm, a thickness of 6.4 mm, and a length of 508mm, each of which was then subjected to two flammability tests throughburner heating.

As set forth in Table 1, the alloys of the present invention satisfiedplastic workability and thermal conductivity, as well as flameretardancy conditions including an ignition point of 550° or more, anignition time of 120 sec or more, a self-extinguishing time of the testspecimen within 180 sec after extinguishment of the burner, and a weightreduction of 10% or less.

As is apparent from the above results, when the total amount of thealloy elements in the alloys of Comparative Examples was 6.5 to 9.45%,ductility deteriorated and extrusion stress was greatly increased orthermal conductivity was decreased.

In contrast, the alloys of Examples of the present invention weremanufactured in the range of 4.4 to 6.0%, and thus superior thermalconductivity and flame retardancy and easy plastic working resulted.

TABLE 1 High-melting- Flammability test (FAA/TC-13/52) point oxide-Weight Extrusion Thermal Zn film-forming Ignition point Ignition timeSelf-extinguishing reduction plastic conductivity Alloy No . (wt %)element (wt %) (TGA, ° C.) (sec) time (sec) (%) working W/m · K  1 C.Ex. 6.2 Y 0.5 487, 503 106, 118 173, 204  9.5, 10.2 Δ  91  2 C. Ex. 6.2Y 1.5 — — — — Broken —  3 C. Ex. 4.5 Ca 2.0 — — — — Stopped —  4 C. Ex.3.0 La 2.0, Si 1.5 — — — — Stopped —  5 C. Ex. 3.0 Sn 4.0 — — — —Stopped —  6 1.0 Sn 3.0, Ca 0.5 590, 648 192, 205 17, 28 0.2, 0.3 ◯ 120 7 3.0 Sn 2.0 601, 623 148, 181  76, 114 2.5, 3.7 ◯ 125  8 2.0 Sn 2.0,Ca 1.0 952, 977 196, 227 12, 23 0.2, 0.3 ◯ 121  9 3.0 Sn 1.0, Ca 0.3, Si0.1 573, 595 172, 211 15, 88 0.2, 0.5 ◯ 130 10 3.0 Sn 2.0, Ca 0.3, Mn0.2 560, 587 193, 214 23, 45 0.3, 0.5 ◯ 118 11 3.0 Sn 2.0, mischmetal1.0 818, 832 183, 197 18, 36 0.4, 0.5 ◯ 121 12 C. Ex. 3.0 Mischmetal2.0, Mn 1.5 — — — — Stopped — 13 C. Ex. 0.91 Al 2.8, Mn 0.2 553, 588101, 109 300, 360 73.8, 80.9 ◯  87 14 C. Ex. 0.75 Al 8.5, Mn 0.2 376,425 78, 96 330, 360 32.3, 45.4 Δ  52 15 C. Ex. — Al 6.2, Y 2.0, Mn 0.4 —— — — Stopped —

FIGS. 3 to 17 show the high-temperature stable phase precipitationdiagrams of the alloys of Examples of the present invention andComparative Examples, in which the high-temperature stable phase beginsto appear at a temperature of at least 430° C., but the high-temperaturestable phase is insufficiently or excessively precipitated inComparative Examples. FIGS. 18 to 23 show the structures of the alloysof the present invention. As shown in the precipitates of the caststructures of FIGS. 18 to 20, the formation of a coarse lamellar phaseat grain boundaries was suppressed, and as is apparent from thestructures of extrudates of FIGS. 21 to 23, the precipitates were finelydispersed during the extrusion.

FIGS. 24 and 25 show the graphs of measurement of thermal conductivityaccording to the present invention. As set forth in Table 1, the alloyof the present invention exhibited thermal conductivity of 115 W/m·K ormore, which is higher than 51 to 54 W/m·K of WE43 or 24 W/m·K of ZE41,as the conventional AZ-based alloy or the currently usefulflame-retardant alloy, and effectively satisfied the flammability test.FIG. 26 shows the flammability test environment of the presentinvention, in which the alloy of the present invention having highthermal conductivity is able to decrease an effect of shortening themelt-down time due to partial heating when exposed to flame, thusincreasing flame retardancy.

[Description of Reference Numerals] 1: mold 2: billet cast material 3:cover 4: shielding gas pipe 5: inlet 6: continuous casting billet stand7: coolant spray nozzle 8: mushy zone 21: lever 22: impeller 41:inductson coil M: motor

What is claimed is:
 1. A magnesium alloy having high thermalconductivity and flame retardancy and facilitating plastic working,comprising: 0.5 to 5 wt % of zinc, 0.6 to 3.5 wt % of tin as ahigh-melting-point oxide-film-forming element, and a remainder ofmagnesium and inevitable impurities, wherein, among the inevitableimpurities, an amount of aluminum or zirconium is controlled to 0.5 wt%.
 2. The magnesium alloy of claim 1, further comprising 1.5 wt % orless of at least one selected from among calcium, silicon, manganese andmischmetal, a total amount of alloy elements being 2.5 to 6.3 wt %.
 3. Amethod of manufacturing a magnesium alloy having high thermalconductivity and flame retardancy and facilitating plastic working,comprising: melting a magnesium ingot in a melting furnace with an airshut-off, thus obtaining a magnesium melt, which is then maintained at atemperature of 680 to 720° C.; melting zinc in the magnesium melt, thusobtaining a magnesium—zinc alloy melt; adding the magnesium—zinc alloymelt with at least one selected from among high-melting-point elements,including tin, yttrium, mischmetal, calcium, silicon and manganese, in aform of a master alloy and then performing mechanical stirring, thusobtaining a magnesium alloy melt; and cooling a mold containing themagnesium alloy melt, thus producing a cast material.
 4. The method ofclaim 3, wherein the melt is cooled in a manner in which the mold issank in a bath containing a coolant or a coolant is sprayed on an outersurface of the mold, and the melt is mechanically stirred duringcooling, whereby solid nuclei of the mushy zone are dispersed.
 5. Themethod of claim 4, wherein the stirring during cooling of the melt isperformed in a manner in which an impeller having a diameter of ⅕ to ⅔of a billet diameter is inserted into the mold so as to performmechanical stirring, whereby the solid nuclei of the mushy zone aredispersed.
 6. The method of claim 3, wherein the cooling is performed ina manner in which the mold is cooled in a bath at a cooling rate of 70to 200° C./min, and the melt in the mold is mechanically stirred duringthe cooling, whereby solid nuclei of the mushy zone are dispersed. 7.The method of claim 3, wherein the cooling is performed in a manner inwhich the magnesium alloy melt is placed in a mold of a continuouscasting device and cooled at a cooling rate of 200 to 900° C./min byspraying a coolant onto an outer surface of the mold and a surface of abillet, and during the cooling, an impeller is inserted into the mold soas to perform mechanical stirring, whereby solid nuclei of the mushyzone are dispersed.